Conventionally, a spark plug used in internal combustion engines such as automotive engines includes an insulator for a spark plug (hereinafter also referred to as simply an “insulator”) formed by an alumina-based sintered body obtained by burning an alumina (Al2O3) type material. The reason why the insulator is formed by an alumina-based sintered body is that the alumina-based sintered body has excellent heat resistance, mechanical strength and withstand voltage characteristics. For burning an alumina-based sintered body, for example, a three-component system sintering aid comprising silicon oxide (SiO2), calcium monoxide (CaO) and magnesium monoxide (MgO) is generally used for the purpose of lowering of a burning temperature and improvement of sinterability.
An insulator is exposed to hot atmosphere of about 500 to 700° C. by the influence of high temperature combustion gas (about 2,000 to 3,000° C.) generated by spark discharge in a combustion chamber of internal combustion engine. For this reason, it is important for an insulator to have excellent withstand voltage characteristics in a range of from room temperature to the above high temperature. In particular, in recent years, increase in occupation space of inlet and exhaust valves in a combustion chamber and 4-valve formation are investigated with increase in power of internal combustion engines, and a spark plug itself tends to be down-sized (small diameter). For this reason, an insulator is required to decrease its thickness, and an insulator having further excellent withstand voltage characteristics even when exposed to hot atmosphere of about 500 to 700° C. is required.
However, in the case of forming an alumina-based sintered body using the three-component system sintering aid described above, the three-component system sintering aid (mainly Si component) is present as a low melting point glass phase in grain boundary of alumina crystal particles after sintering. Therefore, when an insulator formed by an alumina-based sintered body is exposed to hot atmosphere of about 700° C., the low melting point glass phase softens by the influence of the temperature, and withstanding voltage characteristics of the insulator are decreased. On the other hand, a low melting point glass phase in an alumina-based sintered body can be decreased by decreasing the amount of the sintering aid added. In this case, an insulator is not densified, or even though densified seemingly, many pores remain in grain boundary constituted by alumina crystal particles, and withstanding voltage characteristics of the insulator are decreased.
The conventional alumina-based sintered body has a low melting glass phase or pores (residual pores) present in grain boundary. In the case of forming an insulator of a spark plug with such an alumina-based sintered body, when high voltage for generating spark discharge is applied to a spark plug in high temperature environment of about 700° C., a low melting point glass phase softens, or electric field concentrates in residual pores, and an insulator may suffer breakdown (spark penetration).
An alumina-based sintered body containing rare earth metals, an insulator formed by the alumina-based sintered body, and the like are proposed for the purpose of preventing decrease in withstand voltage characteristics and/or breakdown. For example, Patent Document 1 describes “an alumina-based sintered body containing at least a rare earth element (hereinafter referred to as “RE”) component, the alumina-based sintered body having a theoretical density ratio of 95% or more.”
Patent Document 2 describes “an alumina porcelain composition using alumina as a main component, comprising a composite sintered body of the alumina as the main component, and a composition of at least one element selected from Al, Si, Mg and rare earth elements, wherein when the amount of alumina as the main component is 100 parts by weight, the amount of the composition of at least one element selected from Al, Si, Mg and rare earth elements is 5 parts by weight or less.”
Patent Document 3 describes “an alumina porcelain constituted of a sintered body having a porosity of 6% by volume or less, the sintered body comprising alumina (Al2O3) having an average particle diameter of 1 μm or less in a sintered composition, and at least one of compound and mixture of at least one of yttria (Y2O3), magnesia (MgO), zirconia (ZrO2) and lanthanum oxide (La2O3), formed in grain boundary and Al2O3.”
However, an insulator is required to have higher withstand voltage characteristics and prevention of breakdown because of the reason why internal combustion engines have high power than ever.    Patent Document 1: JP-A-2001-2464    Patent Document 2: WO05/033041 pamphlet    Patent Document 3: JP-B-7-12969 (1995)